bpo-30406: Make async and await proper keywords (#1669)

Per PEP 492, 'async' and 'await' should become proper keywords in 3.7.
diff --git a/Lib/lib2to3/pgen2/token.py b/Lib/lib2to3/pgen2/token.py
index 1a67955..7599396 100755
--- a/Lib/lib2to3/pgen2/token.py
+++ b/Lib/lib2to3/pgen2/token.py
@@ -62,10 +62,8 @@
 COMMENT = 53
 NL = 54
 RARROW = 55
-AWAIT = 56
-ASYNC = 57
-ERRORTOKEN = 58
-N_TOKENS = 59
+ERRORTOKEN = 56
+N_TOKENS = 57
 NT_OFFSET = 256
 #--end constants--